Closed loop systems

Closed loop systems are the most common. When properly installed, they are economic, efficient, and reliable. Water (or a water and antifreeze solution) is circulated through a continuous buried pipe. The length of loop piping varies depending on ground temperature, thermal conductivity of the ground, soil moisture, and system design.

Horizontal closed loop systems or Ground Source Heat Pumps rely on ground temperature. As almost half of the energy from the Sun is absorbed and stored by the ground, due to its heat retaining capacity a stable heat balance system emerges. It provides a relatively constant temperature, as the ground below the frost line is not much affected by surface temperature. Ground Source Heat Pumps tap this energy storage charged and levelled by the Sun through millions of years. Horizontal closed loop installations are generally most cost-effective for small installations, particularly for new construction where sufficient land area is available. These installations involve burying pipe in trenches dug with backhoes or chain trenchers.

Vertical closed loops are preferred in many situations. For example, most large commercial buildings and schools use vertical loops because the land area required for horizontal loops would be prohibitive. Vertical loops are also used where the soil is too shallow for trenching. Vertical loops minimize the disturbance to existing landscaping. Vertical loop systems differ from Ground Source Heat Pumps in a way that they use the heat provided by the earth’s crest, so geothermal gradient is a decisive feature with regard to their operation.

Pond closed loops are a special kind of closed loop system. Where there is a pond or stream that is deep enough and with enough flow, closed loop coils can be placed on the pond bottom. Fluid is pumped just as for a conventional closed loop ground.

Open loop systems

An open loop is a loop established between a water source and a discharge area in which the water is collected and pumped to a GHP then discharged to its original source or to another location. The piping for such configuration is open at both ends and the water is utilized only once.

3.            IMPLICATIONS

3.1        Energy Efficiency Implications

There are three factors determining the efficiency of geothermal energy production:

-        recovery of water reserves,

-        suitable water reservoirs,

-        geothermal gradient.

Geothermal energy can be best utilized with cascaded heat extraction. For example, thermal water of 90 - 60 °C can be used for district heating, 60 - 35 °C for greenhouses, while the next step could be the provision of hot water health spa.

Local exploitation can be significant and economic where

-        the geological endowments are favourable,

-        there are good formations permeable to water that can be found not too deep but at a relatively high temperature.

Even if the aforementioned conditions are met, geothermal energy production can still be hindered. When the geothermal fluid is moving upward the pressure and sometimes even the temperature drops, and this way scale is produced. Scale might cause blockage and reduced performance, especially if the water carries sand or other sediments. Chemicals can remove scale, but in open loop systems salt and chemicals in the effluent water will make catchment waters unsuitable for irrigation, and may also result in salinization of the soil. On the one hand this makes re-injection very important, and on the other one unnecessary water extraction should be avoided.